1. Field of the Invention
This invention relates to an injection mold, and more particularly to an injection mold for molding a glass-like tubular article having a closed end.
2. Description of the Prior Art
In plastic injection molding of a glass-like tubular article with a closed end, there is encountered a problem that, when the tubular article is ejected from the core of the movable mold half, vacuum builds up between the top surface of the core and the closed end of the article as the article is moved away from the core. If the article is forced away from the core against the force of the vacuum, the article may suffer breakage or the inner surface of the article may be scratched on the core. Sometimes resin material is scraped off from the inner surface of the article in the form of resin particles and the resin particles adhere to the inner surface of the article under the influence of electrostatic attraction force, and the resin particles may subsequently mingle with material loaded in the article. In order to avoid this problem, air must be fed in between the inner surface of the article and the outer surface of the core when ejecting the article from the core.
There has been known an injection mold in which a telescopic mold piece is inserted into a recess which is formed on the top of the core and which has a diameter slightly larger than the telescopic mold piece so that a small space is formed between the outer peripheral edge of the mold piece and the inner peripheral edge of the recess, and the core is provided with a communicating passage for communicating the lower surface of the mold piece with the atmosphere. In the ejection air is introduced between the article and the core through the small space to prevent a vacuum from being built up. The width of the space must be very narrow so that molten resin injected into the cavity does not flow into the space and typically should be not larger than 2/100 mm. Further, since it is very difficult to insert the telescopic mold piece into the recess on the top of the core precisely coaxially therewith, the mold piece and the recess are generally sized so that the width of the space does not exceed 2/100 mm at any position of the mold piece relative to the recess, and accordingly the width of the space is normally substantially smaller than 2/100 mm. The narrow width of the space increases, the flow resistance of the air and, though depending on the ejecting speed, sufficient air cannot be fed in between the article and the core to prevent a vacuum from being built up therebetween during ejection.
The difficulties in the known injection mold may be overcome by feeding air through the space between the mold piece and the recess under pressure. However, this approach is disadvantageous in that in the case of a multicavity mold it is very difficult to control the air pressure to be fed to each core. That is, if one of the cores is fed with air at an excessively high pressure, the article carried on the core can be thrown away from the core into the corresponding cavity in the stationary mold half, and if the mold is subsequently closed with the article remaining in the cavity, the mold can be damaged.